The fiber optics telecommunications field includes such technologies as fiber optical cables and fiber optical networks. Fiber optical networks carry a great variety of everyday information signals, such as conversations, data communications (e.g., fax messages), computer-to-computer data transfers, cable television, the Internet, and so forth. Such information signals are transported between cities as well as from place to place within cities. Due to the rapidly increasing amounts of such communication traffic, the increased capacity of fiber optical cables is more and more necessary, compared to the lower capacities of older metallic wire cables.
An optical fiber cable is typically composed of a bundle of individual optical fibers. One single optical fiber can carry thousands of data and communication signals on a single wavelength of light. That same single optical fiber can also carry multiple wavelengths of light, thus enabling it to carry many, many multiple optical signals at the same time. Each wavelength alone can carry data that transfers at a rate over 10 Gbit/s.
To maintain communications over such optical networks, it is necessary to perform a variety of sensitive analyses, such as measuring the optical power, wavelength, and the optical signal-to-noise ratio of the optical signals at each of the wavelengths traveling through the optical fiber. Such analysis is carried out by an analytical tool called an optical spectrum analyzer (“OSA”). The OSA performs optical spectrum analysis (also referred to as “OSA”), which, as indicated, is the measurement of optical power as a function of wavelength.
OSA applications include testing laser and/or light-emitting diode (“LED”) light sources for spectral purity and power distribution, monitoring an optical transmission system of a wavelength division multiplexing (“WDM”) system for signal quality and noise figures, testing transmission characteristics of various optical devices and components, and so forth.
OSA is typically performed by passing an optical signal to be analyzed through a tunable optical filter. “Tunable” means that the filter can be adjusted to resolve or pick out the individual components (wavelengths) of the optical signal.
The optical resolution of an OSA is the minimum wavelength spacing between two spectral components that can be reliably resolved. To achieve high optical resolution, the filter should have a sufficiently narrow 3-dB bandwidth (“BW”). Additionally, for many measurements the various spectral components to be measured are not of equal amplitudes, in which case the BW of the filter is not the only concern. Filter shape, which is specified in terms of the optical rejection ratio (“ORR”) at a certain distance (e.g., ±25 GHz) away from the peak of the transmission, is also important. An example is the measurement of side-mode suppression of a distributed feedback (“DFB”) laser and measuring the optical-signal-to-noise (“OSNR”) of the various wavelength channels in WDM optical communications systems.
Three basic types of filters are widely used to make OSAs: diffraction gratings, Fabry-Perot (“FP”) filters, and Michelson interferometers. A tunable FP filter (“TFPF”) has many advantages for OSA. Principal among these are its relatively simple design, small size, fast speed, ease of control, and its great sensitivity for distinguishing optical signals that are very closely spaced (i.e., signals that have frequencies or wavelengths that are very nearly the same.)
The wavelength scanning range of the FP filter OSA is defined by its free spectrum range (“FSR”). For the same finesse value, the FP filter's BW is proportional to its FSR, which means the larger the FSR, the poorer the resolution. Thus for many FP filter OSA applications, there are two major challenges. One challenge is to achieve a very high dynamic range for optical signal-to-noise-ratio (“S/N”) measurements (for example, for characterizing a dense wavelength division multiplexing (“DWDM”) system). The other is to achieve a very wide scanning range of wavelengths (for example, from 1260 nm to 1630 nm) while maintaining a sufficiently narrow bandwidth.
A FP filter OSA has a limited wavelength scanning range as defined by the filter's FSR, which is the spectral separation between adjacent Fabry-Perot orders (optical orders) that can be tuned without overlap. The FSR is related to the full-width half-maximum of a transmission band by the FP filter's finesse. FP filters with high finesse desirably show sharper transmission peaks with lower minimum transmission coefficients.
A FP filter can be fabricated to have a very narrow 3-dB BW, thus providing very good spectral resolution. The FP filter's FSR is proportional to the mathematical product of the filter's BW and its finesse. If the BW becomes smaller, the finesse needs to be larger to maintain the same FSR. For the same finesse value of the FP filter, the larger the FSR, the larger the BW. This is not desirable in many applications since the larger the BW, the poorer the spectral resolution. Thus, in using a FP filter to construct an OSA, there is yet another challenge, which is that the filter's FSR will limit the filter's wavelength scanning range.
A need thus remains for methods and apparatus that can scan a broad wavelength range, that is substantially larger than the FP filter's inherent FSR, while maintaining a sufficiently narrow BW and thus excellent spectral resolution. In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.